1. Field of the Invention
The present invention relates to a high definition television (HDTV) receiver, and more particularly, to an HDTV receiver employing a single decoder as both an optimal trellis decoder and a partial response trellis decoder.
2. Discussion of the Related Art
A recently proposed transceiver of the grand alliance (GA) HDTV system has an 8 vestigial side band (VSB) transmitting system. In the 8 VSB transmitting method, input data are set for a trellis coding, and then converted into an 8 level signal, which is then transmitted in the VSB method. That is, when two-bit data are input, the trellis encoder does not code the higher bit, but 1/2 convolutional codes the lower bit in order to make it into two bits, thereby outputting three bits in total. The three bits are trellis coded modulated (TCM), which are mapped into highly error-correctable levels, namely, 8 levels. The TCM is the modulation of the convolutional coded signal, that is, a channel coding method capable of obtaining a signal-to-noise ratio of over about 3 dB without a loss of the channel bandwidth. Meanwhile, the signal transmitted in the VSB method is decoded using a trellis decoder. If there is an NTSC broadcasting station having a co-channel near the HDTV broadcasting station, the co-channel NTSC interference occurs. In this case, a comb filter, an NTSC rejection filter, is used for removing the co-channel NTSC interference. Accordingly, different decoders are used for decoding the data when using the comb filter and when not using it.
FIG. 1 shows a block diagram showing the relations between a trellis encoder 21, comb filter 23, optimal trellis decoder 25 and partial response trellis decoder 27. FIG. 2 shows a detailed block diagram of the trellis encoder of FIG. 1.
In FIG. 2, the trellis encoder 21 includes a precoder 21-1, a convolutional encoder 21-2, and a TCM mapper 21-3. The higher bit X2 of the 2 bit signals X2, X1 input to the trellis encoder 21 is differentially encoded in the precoder 21-1 so that the partial response trellis decoder 27 can exactly decode the higher bit X2 which is not encoded, and then input to the TCM mapper 21-3 Z2. One lower bit X1 becomes 2 bits Z1, Z0 via the convolutional encoder 21-2, which are input to the TCM mapper 21-3. When the higher 1 bit X2 is input to the precoder 21-1, the summer 21-1a sums the input higher bit X2 and the data previously stored in the delayer 21-1b and then outputs it. At this time, "0" is output when the two values are the same, and "1" is output when the two values are different. Therefore, the trellis decoder can exactly recognize the value of the higher one bit X2 if only performing a hard decision upon the input data.
As illustrated in FIG. 3, the TCM mapper receiving three bits Z2, Z1, Z0 transmits the 8 level voltage signal corresponding to the input data as illustrated in FIG. 3. Therefore, when the trellis encoder 21 of the transmitting part transmits the 8 level signal, the trellis decoders 25, 27 of the receiving part decode it into the two bit signals X2, X1 prior to encoding by the trellis encoder 21. Here, because the co-channel NTSC interference occurs in the area of the adjacent HDTV broadcasting station having the co-channel, the trellis decoder removes the NTSC interference using the comb filter 23 made up with a 12 symbol delayer 23-1 for delaying the value output from the TCM mapper 21-3, and the summer 23-2 for obtaining the difference of the value output from the current TCM mapper 21-3. Here, when the output of eight level from the TCM mapper 21-3 of the trellis encoder 21 passes the comb filter 23, the output is converted into 15 levels as illustrated in FIG. 4. Accordingly, the decoded data should be decoded using each different decoder when both using the comb filter 23 and not using it. That is, if there is no interference by the NTSC signal having the co-channel in the received signal, the signal is decoded by four state optimal trellis decoder 25. If there is interference by the NTSC signal having the co-channel, the signal is decoded by the 8 state partial response trellis decoder 27 via the comb filter 23. Here, the optimal trellis decoder 25 performs decoding using the signal mapper as same as FIG. 3.
The state transition is illustrated in FIG. 5. The signal group is shown in FIG. 6.
Here, the capital reference characters S0, S1 indicate memory states of the trellis encoder 21. The small letters s0, s1, s2, s3 indicate signal groups. That is, s0={-7, 1}, s1={-5, 3}, s2={3, 5}, s3={-1, 7}, and the euclidean distance values of respective groups are d0, d1, d2, d3.
Meanwhile, the partial response trellis decoder 27 performs the decoding operation using the signal passing through the comb filter 23 as illustrated in FIG. 4. The state transition is shown in FIG. 7, and the signal group is shown in FIG. 8. Likewise, the capital letters S1, S2, S3 indicate the memory states of the trellis encoder 21. The small letters s0, s1, s2, s3, s4, s5, s6 indicate the signal groups. That is, s0={-14, -6, 2}, s1={-12, -4, 4}, s2={-10, -2, 6}, s3={-8, 0, 8}, s4={-6, 2, 10}, s5={-4, 4, 12}, s6={-2, 6, 14}.
The above-mentioned HDTV receiver should have two trellis decoders such as the optimal trellis decoder and the partial response trellis decoder in accordance with the operation of the comb filter whether there is the NTSC signal having the co-channel or not. Therefore, twice as much hardware is required for respectively realizing the two kinds of trellis decoders, and the process is complicated because of the two kinds of control signals to process those outputs.